Organic thin-film transistors are formed as thin films of an organic semiconductor, dielectric and electrodes on a substrate. A wide variety of substrates can be used, including traditional semiconductor substrates and other substrates such as glass, polymers, etc. Depending on the substrate selected and the materials used for electrodes, organic thin film transistors can be completely transparent when transparent semiconductors and electrodes are used. Transparency is advantageous, for example, in display applications. Organic thin film transistors have a wide range of applications, e.g., displays, optoelectronics, logic circuits, and sensors. Carrier mobility and other performance characteristics of organic semiconductor devices are presently low compared to single-crystal inorganic semiconductors, however organic thin film transistors offer considerable advantages in lower processing temperature, larger area fabrication, and lower cost manufacturing.
Some materials used in organic thin film transistors have other advantages over inorganic single-crystal semiconductors. Semiconducting CuPc thin-films, for example, have high chemical stability and exhibit substantial responses to chemical analytes as chemiresistors. Ultrathin organic thin film transistors are of interest for studying intrinsic electrical properties of organic materials and also for their potential to have reduced bias stress compared to standard organic thin film transistors and enhanced sensitivity in chemical field-effect transistors (ChemFETs). ChemFETs are organic thin film transistors whose output characteristics are sensitive to the presence of analytes via changes in the channel mobility and/or threshold voltage induced by analyte chemisorption onto the channel materials.
Chemical sensing is a critical process in a large number of everyday household, industrial, military, and scientific processes. A chemical sensor that can indicate the presence of a chemical of interest is useful to provide warnings, such as to indicate an unacceptable level of carbon monoxide or to provide a warning regarding the presence of an explosive vapor or a chemical warfare agent or vapors from secondary sources such as household molds. Similarly, chemical sensors can provide information on the presence or absence of a particular chemical in a process control scheme. The presence or absence of a gas can provide feedback used to control a wide range of industrial processes. In the area of scientific research, many instruments including, for example, chromatography instruments benefit from sensitive chemical detectors.
Sensitivity is a critical aspect of chemical sensors. More sensitive sensors can detect lower levels of chemical agent. Accordingly, there is great interest in producing highly sensitive chemical sensors. Early warning regarding levels of sensed chemicals, faster control of processes responsive to particular levels of sensed chemicals, and better detection in difficult environments are achieved as sensitivity increases. Some particular example applications of interest in the art will now be discussed.
One application of interest is the detection of ultra-trace amounts of explosives and explosive-related analytes. Such detection is of critical importance in detecting explosives in a number of civilian and military or security applications, e.g., mine fields, military bases, remediation sites, and urban transportation areas. Low-cost and portability have clear additional advantages to such sensor applications.
In security applications, chemical sensors are preferable to other detection devices, such as metal detectors, because metal detectors frequently fail to detect explosives, such as those in the case of the plastic casing of modern land mines. Similarly, trained dogs can be both expensive and difficult to maintain in many desired applications. Other detection methods, such as gas chromatography coupled with a mass spectrometer, surface-enhanced Raman Spectroscopy, nuclear quadrupole resonance, energy-dispersive X-ray diffraction, neutron activation analysis and electron capture detection are highly selective, but are expensive and not easily adapted to a small, low-power package for broad distribution.
A particular type of chemical sensor that has been investigated is an organic thin film transistor that has its conduction channel affected in the presence of a chemical analyte. The general principal has been demonstrated, while typical efforts have not demonstrated a useful level of sensitivity. Example transistor chemical sensors are disclosed in the following articles. Torsi, et al., Sens. Actuators, B 67, 312 (2000). The channel material in Torsi et al was 1,4,5,8-naphthalene tetracarboxyl dianhydride. Channel thickness in the chemical sensor was 500 Å (>50 MLs); Crone, et al., Appl. Phys. Lett. 78, 2229 (2001) discloses a sensor with channel materials: of di-dodecyl a-6T. The thickness was 100-1000 Å (10-100 MLs). Someya, et al, Appl. Phys. Lett. 81, 3079 (2002) discloses channel materials: of DHa4T with thickness: 150 Å (15 MLs). Zhu, et al, Appl. Phys. Lett. 81, 4643 (2002), discloses channel materials of pentacene. The thickness was 500 Å (50 MLs).
Studies of the charge transport process in organic thin film transistors have shown that carriers conduct primarily through the first 1-5 MLs above the gate dielectric. G. Horowitz, J. Mater. Res. 19, 1946 (2004). Many prior organic thin film transistor sensors have not recognized or taken advantage of the charge carrier mechanism, having channel layers that are 10 monolayers or more, and typically 50 monolayers or more. Despite intensive study of organic thin film transistors in chemical sensing, the transduction mechanism is not fully understood.
Another issue that impacts and limits usefulness of organic thin film transistor sensors is baseline drift. With static gate bias operation, even encapsulated organic thin film transistors exhibit a large bias stress effect (BSE). This bias instability has been observed on organic thin film transistors fabricated with a wide range of active materials and is associated with charge trapping in the organic film. Substantially delaying the time between measurements by hundreds of seconds reduces the BSE, but such a time delay is too long for many chemical sensing measurements. Baseline voltage compensation has also been considered, but simple baseline voltage compensation methods fail to reduce BSE.
As mentioned above, the conduction channels in organic thin film transistors have carriers conducting primarily through a few monolayers (MLs) above the gate dielectric. Due to this conduction mechanism, the contact between the electrode and the organic thin film layer and substrate significantly affects the carrier transport behavior of organic films. Conventional thin film electrode fabrication techniques can produce contact resistances that are high and that produce a large number of inoperable or low performance devices as a percentage of devices that are being fabricated
Another issue of concern in the manufacture and uses of organic thin film transistors is the long term stability and device integrity in ambient operating conditions. Oxygen and humidity are known to produce instability in organic thin film transistors, which can have performance that suffers from degradation or aging.